Methods and devices for coating inner surfaces of tubes are known in the art. For example, coating technology used for tubes employed in oil and gas industry includes methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), electroplating, flame spray, sol-gel, and polymer lining.
However, all these methods relate to application of anti-corrosive and wear-resistant coatings onto inner surfaces of metal tubes and are not applicable for tubes made from plastic material, in particular, to those plastic tubes which are used in pharmaceutical or food industry. This is because processes for application of coatings onto plastic tubes require low temperature.
An example of an apparatus and method suitable for coating inner surfaces of plastic tubes are disclosed in U.S. Pat. No. 4,897,285 issued in 1990 to R. Wilhelm. This patent describes a microwave plasma apparatus and method for depositing a coating of a prescribed material onto the internal surface of a tube by means of a reaction of a vapor phase providing a prescribed material on the internal surface of a tube. In the aforementioned vapor-phase reaction (a) an atmosphere at a pressure less than atmospheric pressure containing the vapor phase is introduced into the tube, (b) microwave energy is beamed into the tube and is propagated along the length of the tube, (c) a magnetic field is generated in a localized area of the tube in which electron cyclotron resonance occurs for the the frequency of the microwaves, and in which, pressure, field strength of the magnetic field and the power of the high-frequency field are selected so that a low-pressure gas discharge results in the area of the magnetic field to trigger the reaction furnishing the prescribed material, and (d) the magnetic field and thus the plasma are transposed along the length of the tube.
In one particular application, the above method and apparatus are intended for low temperature, plasma assisted, chemical vapor deposition of a thin film coating onto the internal circumferential wall of the elongated tubular member fabricated of a nylon material so as to hermetically seal same from gaseous permeation or other ambient conditions. Nylon tubes which are hermetically sealed with a coating of silicon oxide, silicon nitride, or silicon oxycarbide, or a similar sealing material, are commercially important for the automotive industry and are used in vehicles, e.g., as hydraulic tubes or air conditioning hoses. Such hoses are suitable for transporting liquid coolants that travel therethrough without loss to the atmosphere. However, the scope of application of the method of the aforementioned invention is limited and is applicable only to those tubes which are made from materials permeable to microwave energy.
U.S. Pat. No. 5,223,308 issued in 1993 to J. Doehler discloses a method for the low temperature, microwave enhanced, chemical vacuum deposition of a thin film material onto the surface of a hollow member by creating a sub-atmospheric pressure condition adjacent the surface to be coated while maintaining the applicator through which microwave energy is introduced at substantially atmospheric pressure.
U.S. Pat. No. 4,349,582 issued in 1982 to H. Beerwald, et al. discloses a method for coating the interior of electrically non-conductive tubes by means a reactive deposition of a gas flowing through the pipe, of the type wherein the gas is disassociated by an electric gas discharge and wherein the deposition occurs simultaneously throughout the total pipe length. Pulse discharges are used having pulse lengths which are so adjusted to the transit time of the gas through the tube that the time period between two successive pulses corresponds to the time which is required for filling the tube with unused gas.
Also known in the art is a great number of patents describing application of coating, including PECVD-applied silicon dioxide (SiO2) layers onto inner surfaces of containers, e.g., plastic bottles for storage of beverages.
The PECVD process is described, e.g., by J. Felts in U.S. Pat. No. 6,180,191 issued in 2001. A PECVD-applied silicon dioxide (SiO2) layer on the inside surface of a PET bottle prevents the ingress of oxygen and the egress of carbon dioxide that would affect the taste of the product and its shelf life. After deposition of a thin silicon oxide coating, the oxygen transmission rate is reduced to 0.076 cc/bottle/day.
The PECVD process first deposits a transparent adhesive layer of nanocrystalline SiO and then a colorless silicon oxide (SiOx) barrier layer having a thickness of 0.01 to 0.1 micron. The SiOx layer improves the oxygen-barrier properties of a bottle more than 10 times, and the SiO2 barrier, specifically, improves this property more than seven times. These barrier improvements remain after hot filling or pasteurization. In addition to the use of PECVD in the food and pharmaceutical industries, application of a PECVD barrier onto the inner surfaces of hollow objects may be used in automotive and piping industries wherein plastic materials such as HDPE are used to replace metals because of their excellent tensile strength and impact properties at temperatures as low as −50° C. and at temperatures as high as 70° C., which match the temperature range in fuel tanks and tube lines. Since HDPE is low in weight and cost, it is competitive with steel. However, HDPE has one drawback, and this is permeation of fuels. In order to overcome this drawback, it is necessary to develop an improved barrier coating suitable for application onto the inner surfaces of HDPE tanks and tubes, especially those designed to contain gasoline, alcohol, or other toxic, corrosive, and health-hazardous materials. Moreover, the same coating system is supposed to serve as an inductive probe to provide quality control of the thickness, uniformity, and integrity of the barrier in the inner surface of the wall after the deposition process. The SiO2 coating has high optical transparency and a markedly improved barrier effect as well as greater tensile strength. Silicon dioxide is nontoxic and does not affect the recycling of PET and HDPE.
The inner container coating of SiO2 provides an excellent gas permeation barrier because of two important properties. First, the coating on the interior surface of the container is not subject to abrasion during shipment and handling when compared to the exterior surface of the container. Second, by forming the coating on the interior surface of the container, degradation of the product within the container from direct interactions between the product and the container is prevented.
Thus, there is a demand for a simple, inexpensive, and reliable process for application of barrier coatings onto the inner walls of polymeric containers. The process should have a fast cycle time to accommodate production demands and be suitable for integration into a bottle-molding production line, such as a Husky molding system with throughput of 15,500 bottles per hour. Further, the barrier coating should have good uniformity, and the barrier-coated polymeric container should be easy to recycle.
A plasma-enhanced chemical vapor deposition (PECVD) coating from a gaseous phase is well known and is used in the semiconductor industry to treat semiconductor wafers. However, a flat substrate such as a semiconductor wafer, which is an object of deposition, can be treated at high temperatures with application of a bias voltage, while in the case of plastic containers or tubes, the material of such containers has a low melting point that cannot withstand high temperatures. Plasma discharge is developed by an RF antenna introduced into the container together with a gas mixture and when the RF antenna is energized, this causes a plasma-chemical reaction that results in generation of silicon dioxide, which is deposited onto the inner walls of the containers in the form of a thin barrier layer of SiO2. The plasma-chemical reaction can be conducted between different silicon-containing gases such as silane or disilane and oxygen-containing gases such as nitrogen dioxide, nitrous oxide, etc. Because of the flammability and explosiveness of silanes, the above process requires special, expensive facilities in the semiconductor industry. The food industry prefers to conduct the processes under less expensive, unpretentious conditions with a safer organosilicon or siloxane and by conducting the plasma chemical-reaction with pure oxygen. The plasma-chemical reaction may also have safe-reaction byproducts, such as CO2 and water. Plasma discharge inside a container decomposes siloxane vapor and breaks off methyl groups. Further, the oxygen oxidizes the condensable siloxane backbone (Si—O—Si) resulting from the organosilicon decomposition, thereby forming a thin film of silicon oxide (SiOx) obtained by plasma-enhanced chemical vapor deposition (PECVD) on the interior surface of the container. Gaseous organosilicon is received, for example, from liquid tetraethylorthosilicate (TEOS). TEOS can be converted into vapor by using a direct liquid injection subsystem DL125-C (a product of MKS Company) that includes a vaporizer that evaporates the liquid into vapor for introducing it into the processing system. Byproducts (CO2 and water) are removed by means of a vacuum system through small holes provided in a bottle holder.
The pure SiO2 barrier, however, presents some disadvantages because it is brittle and can be torn during bending and squeezing. In order to enhance durability of the coating, a double-layer coating is preferred wherein the first thin layer is a layer of nanocrystalline SiO2 deposited on the plastic wall. This first layer blocks the porosity of plastic and simultaneously improves adhesion to plastic of the next thick layer of amorphous SiO2 intended for contact with the liquid. This layer increases chemical resistance of the wall to aggressive species and simultaneously reinforces the barrier layer to prevent rupture of the film.
On the other hand, known in the art is ICP discharge, which is used as a source of light and has been used as a source of light for some time. An ICP discharge has been described and analyzed in literature, such as in articles by R. B. Piejack, V. A. Godyak, and B. M. Alexandrovich titled “A simple analysis of an inductive RF discharge,” Plasma Sources Sci. Technol. 1, 1992, pages 179 to 186, and “Electrical and Light Characteristics of RF-Inductive Fluorescent Lamps,” Journal of the Illuminating Engineering Society, Winter 1994, pages 40 to 44. An ICP light source comprises a vacuum vessel, an inductive coupling system immersed in the vessel, and a high-frequency power source. In the initial stage of operation of inductively coupled plasma, an electrical field (E field) ionizes the fill in the gas-filled volume, and the discharge is initially a characteristic of an E discharge. Once breakdown occurs, however, an abrupt and visible transition to the H discharge occurs. Inductively coupled plasma works on the principal of producing an electric field in a body of gas by means of electromagnetic fields induced by oscillating current in the vicinity of the gas.
When the fields induced in gas are strong enough, the gas can break down and becomes ionized in order to generate plasma. Such plasma has been used for a number of applications ranging from fluorescent lighting to plasma treatment of semiconductor wafers. During operation of an inductively coupled discharge, both E and H discharge components are present, but the applied H discharge component provides greater (usually much greater) power to the plasma than the applied E discharge component.
The inductively coupled plasma has been created by either wrapping a solenoid coil around a glass or quartz tube containing gas (“helical induction”) or by placing such a solenoid or spiral within the volume of gas itself (“immersed induction”). In a typical approach, an RLC circuit created by the inductive coil and a matching circuit are tuned to resonance and develop high currents on the coil. An alternating electromagnetic field induced within the gas volume creates a conductive plasma discharge having characteristics similar to secondary winding of a transformer, with a portion of the current through the discharge being converted to light. Lighting devices using immersed induction are described by Hewitt in U.S. Pat. No. 966,204, issued Aug. 2, 1910. Generation of light requires high plasma density in the center of a vessel so that the flat spirals, or solenoids, are immersed in a vacuum bulb having axial symmetry. However, use of axially symmetric antennas is not applicable to elongated containers, e.g., bottles, since they cannot generate plasma having high and uniform density near the inner walls of containers.
An example of use of capacitively coupled plasma for deposition of a barrier coating layer onto inner surfaces of bottles is disclosed in German Patent DE 3,908,418, by H. Grunwald, issued Sep. 20, 1990. This patent describes a system designed for plasma-assisted film deposition or treatment of hollow containers and comprises a capacitively coupled plasma system to drive a low-pressure gas discharge within the form. Such a system also has disadvantages, including a potentially lower deposition and treatment rate for mass-produced applications. Similar to other capacitively coupled plasma systems, the system of the aforementioned invention uses high plasma sheath energies that may result in excess heating of sensitive plastic container walls resulting in container damage. This design is also complicated and may require expensive and regular maintenance caused by film deposition on power-coupling components.
Also known in the art is the use of apparatus for coating the inner walls of containers, such as bottles, by means of deposition from inductively coupled plasma (see, e.g., U.S. Pat. No. 5,521,351 issued in 1996 to L. Mahoney). This invention relates to inductively coupled plasma generated within the interior of a hollow form held within a vacuum chamber enclosure by using a radio frequency coil mounted within the vacuum chamber around the outer surface of the container and closely conforming to the shape of the hollow container. The interior of a hollow form having complex shapes can be treated using two or more coils arranged to treat distinct portions of the form, and the shape of the coils and the manner in which power is supplied to the coils can be selected to control spatial distribution of the plasma within the hollow form.
A main drawback of all apparatuses and methods for application of coatings onto the inner surfaces of containers known to the inventor is a non-optimal direction of the magnetic fields generated by the antenna coils. RF power applied to these coils provides RF current that generates an axial magnetic field. Therefore, plasma density in such systems is distributed so that maximum plasma density is concentrated in the vicinity of the axis but minimum plasma density is close to the inner wall of the container, when the antenna is used for plasma-enhanced chemical vapor deposition of a barrier layer onto the inner walls of the aforementioned container. Coating of the walls in such a system has a low throughput rate. In other words, the existing antennas of apparatus for treating inner surfaces of containers have a geometry that does not produce plasma fields that match the inner profiles of containers.
In order to overcome the drawbacks inherent in RF antennas with an axial magnetic field, the inventor herein developed a new and unique transversal RF antenna that generates a magnetic field normal to the axial direction of the RF antenna and hence normal to the inner walls of the container into which the RF antenna is inserted. The apparatus and method of the aforementioned invention are described in pending U.S. patent application Ser. No. 12/152,064 filed by the same applicant on May 12, 2008. According to the above invention, an antenna assembly consists of a holder which supports a transversal RF antenna with a plurality of multi-turn coils connected in series or in parallel and intended for generation of an inductively coupled plasma discharge inside a container with high plasma density in vicinity of the container's inner walls. The aforementioned discharge is used for inducing in the container a plasma chemical reaction between oxygen and organosilane with resulting deposition of the reaction product in the form of silicon dioxide onto the inner walls of the container for forming a fluid-impermeable barrier layer. A specific feature of the antenna is that it generates a magnetic field transversal to the longitudinal axis of the antenna, i.e., normal to the container's walls, where a maximal electric field, maximal plasma density and, correspondingly, maximal rate of deposition of silicon dioxide on the wall are achieved.
However, the antenna assembly of the aforementioned invention is intended for application of barrier layers onto inner surfaces of containers and is not directly applicable for application of protective layers onto inner surface of elongated tubes. This is because a container has a bottom and can be easily sealed prior to generation of vacuum by an antenna holder. Furthermore, the volume of the container is small and the antenna inserted into the container has essentially the same length as the container itself so that treatment can be carried out over the entire length with deposition of the layer on the entire surface of the container at once, which cannot be done efficiently and with high uniformity of coating inside a long tube or tube.